RNA plays an important role in cellular regulation -- either by its presence in active form or by its total absence. It has been known for several years that transcription of DNA does not necessarily lead to productive, mature RNA molecules. Cleavage of these RNA molecules often is required for the RNA molecules to mature or to acts as an intermediate in other processes (e.g., priming of DNA replication). These cleavage events are a subset of a general maturation pathway known as RNA processing. Work of this Intramural Research Project is concerned with two types of RNA processing, generation of RNA primers for DNA replication and ribosomal RNA processing in higher eukaryotes. We have demonstrated that the requirement for ribonuclease H for cell growth can be supplanted by genes thought to be normally involved in recombination. These results suggest that RNaseH is required for normal DNA replication but a second, poor pathway for DNA replication occurs via recombinagenic activity. Ribosomal RNA processing has been studied by studying an enzyme which we previously described and implicated as being involved in rRNA processing. A small nucleolar RNA (U3) seems to be easily separated from the ribonuclease on DEAE column chromatography. In vitro transcription systems from mouse have been used to synthesize hybrid chick-mouse rRNA. To date, no in vitro processing has been observed in this system.